Well, I only have 2, so I'm not best placed to comment really, but I would reiterate what Tinuviel says about working them together where possible, and even with two, I have a couple of strategies to stop me going insane!
One is to work them both on the same thing. So, whatever, DD1 (13) does, DD2(10) does too, just at a lower level, IYSWIM. It seems to work well and I've been really surprised at what DD2 can actually cope with.
One is to concentrate on something with one and not give a about what the other one is doing! Even if they only had fifteen minutes each with you, that's fifteeen minutes more of one to one than they'd be getting at school! So I will often work with one on something they are stuck on, and frankly, who cares what the other is doing! TV, DS, climbing trees, their time to have one to one with you will come later. And actually, I'm always amazed at how productive what they choose to do on their own actually is! You could work this just as well with 4.
The last one is to set them something to do, send them off to do it and tell them to come back for me to check it once it is all done. This might not work so well if they didn't have their own rooms, mind you. Also it depends on character a bit! DD2 invariably presents me with 12 pages of work half an hour later (and has the rest of the day completely free), whereas DD1 can still be languishing in her room at 5pm! Still, if I am busy with other stuff I have to do, it works well enough as a method.